Bioerodible endoprostheses and methods of making the same

ABSTRACT

A bioerodible endoprosthesis erodes to a desirable geometry that can provide, e.g., improved mechanical properties or degradation characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC §119(e) to U.S.Provisional Patent Application Ser. No. 60/844,966, filed on Sep. 15,2006, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to bioerodible endoprostheses, and to methods ofmaking the same.

BACKGROUND

The body includes various passageways such as arteries, other bloodvessels, and other body lumens. These passageways sometimes becomeoccluded or weakened. For example, the passageways can be occluded by atumor, restricted by plaque, or weakened by an aneurysm. When thisoccurs, the passageway can be reopened or reinforced with a medicalendoprosthesis. An endoprosthesis is typically a tubular member that isplaced in a lumen in the body. Examples of endoprostheses includestents, covered stents, and stent-grafts.

Endoprostheses can be delivered inside the body by a catheter thatsupports the endoprosthesis in a compacted or reduced-size form as theendoprosthesis is transported to a desired site. Upon reaching the site,the endoprosthesis is expanded, e.g., so that it can contact the wallsof the lumen.

The expansion mechanism may include forcing the endoprosthesis to expandradially. For example, the expansion mechanism can include the cathetercarrying a balloon, which carries a balloon-expandable endoprosthesis.The balloon can be inflated to deform and to fix the expandedendoprosthesis at a predetermined position in contact with the lumenwall. The balloon can then be deflated, and the catheter withdrawn fromthe lumen.

It is sometimes desirable for an implanted endoprosthesis to erode overtime within the passageway. For example, a fully erodible endoprosthesisdoes not remain as a permanent object in the body, which may help thepassageway recover to its natural condition. Erodible endoprostheses canbe formed from, e.g., a polymeric material, such as polylactic acid, orfrom a metallic material, such as magnesium, iron or an alloy thereof.

SUMMARY

The invention relates to bioerodible endoprostheses and methods ofmaking the endoprostheses. The endoprostheses can be configured to erodein a controlled and predetermined manner in the body.

In one aspect, the invention features an endoprosthesis including abody, which includes a cross section in the X-Y plane and extends alonga z-axis; and an erosion modifying material provided on the surface ofthe body which controls erosion to form a predetermined geometry suchthat, after erosion of at least about 50 percent of the area of the bodyin the X-Y plane, at least one initial dimension of the initial geometryis maintained. The body has an initial geometry in the X-Y planecharacterized by initial dimensions.

In another aspect, the invention features an endoprosthesis including abody, which includes a bioerodible metal material, has a cross-sectionin the X-Y plane, and extends along a z-axis; and a predeterminedgeometry after erosion of at least about 50 percent of the area of thebody in the X-Y plane, the predetermined geometry is in the shape of anI, an X, an interdigitated structure, a radially lobed structure, or aconvex structure. The body has an initial geometry in the X-Y planecharacterized by initial dimensions.

In yet another aspect, the invention features an endoprosthesisincluding a body, which includes a bioerodible metal material, has across-section in the X-Y plane, and extends along a z-axis; and anerosion modifying material provided on the surface of the body whichcontrols erosion to form a predetermined geometry, the modifyingmaterial being provided in a pattern of at least three separate regionsin the X-Y plane. The body has an initial geometry in the X-Y planecharacterized by initial dimensions.

Embodiments can include one or more of the following features.

The initial dimension can be maintained after erosion of at least about55 percent (e.g., at least about 60 percent, at least about 65 percent,at least about 70 percent, at least about 75 percent, at least about 80percent, at least about 85 percent, at least about 90 percent, at leastabout 95 percent) of the area of the body in the X-Y plane. The initialdimension maintained can correspond to the maximum dimension of theinitial geometry. In some embodiments, the initial geometry is square orrectangular. In some embodiments, the initial geometry is circular,ovaloid, or elliptical. The ratio of the maximum initial dimensions inthe X-Y plane can be between about 2:1 and about 1:2 (e.g., about 1:1).

The predetermined geometry can be an I shape. In some embodiments, theends of the I correspond to abluminal and adluminal sides of theendoprosthesis. The predetermined geometry can be an X shape. In someembodiments, the predetermined geometry is an interdigitated geometry.In some embodiments, the predetermined geometry is a radially lobedstructure. In some embodiments, the predetermined geometry is a convexstructure. In some embodiments, the predetermined geometry is square orrectangular. The predetermined geometry can extend substantially thefull extent of the body in the Z direction.

The bioerodible material can include a magnesium, calcium, aluminum,strontium, zirconium, zinc, manganese, iron, nickel, copper, cobalt, arare earth element, and/or alloys thereof.

The erosion modifying material can include a polymer, a ceramic, anoxide, a metal, an alloy, and/or a composite. The erosion modifyingmaterial can be a layer. In some embodiments, the layer has varyingthickness. The thickness can vary in the X and/or Y direction. Thethickness can vary in the Z direction. The thickness can vary along thelength of the endoprosthesis. The erosion modifying material can beprovided on the entire surface of the body. The erosion modifyingmaterial can include multiple materials at select locations to controlthe erosion of the body.

In some embodiments, the body can be a strut. The endoprosthesis can beformed of a plurality of struts arranged in the general form of a tube.The endoprosthesis can be balloon expandable.

Embodiments may have one or more of the following advantages.

The endoprostheses may not need to be removed from a lumen afterimplantation. The endoprostheses can have a low thrombogenecity and highinitial strength. The endoprostheses can exhibit reduced spring back(recoil) after expansion. Lumens implanted with the endoprostheses canexhibit reduced restenosis. The rate of erosion of different portions ofthe endoprostheses can be controlled, allowing the endoprostheses toerode in a predetermined manner and reducing, e.g., the likelihood ofuncontrolled fragmentation and embolization. For example, thepredetermined manner of erosion can be from an inside of theendoprosthesis to an outside of the endoprosthesis, or from a first endof the endoprosthesis to a second end of the endoprosthesis. Thecontrolled rate of erosion and the predetermined manner of erosion canextend the time the endoprosthesis takes to erode to a particular degreeof erosion, can extend the time that the endoprosthesis can maintainpatency of the passageway in which the endoprosthesis is implanted, canallow better control over the size of the released particles duringerosion, and/or can allow the cells of the implantation passageway tobetter endothelialize around the endoprosthesis.

An erodible or bioerodible endoprosthesis, e.g., a stent, refers to anendoprosthesis, or a portion thereof, that exhibits substantial mass ordensity reduction or chemical transformation, after it is introducedinto a patient, e.g., a human patient. Mass reduction can occur by,e.g., dissolution of the material that forms the endoprosthesis and/orfragmenting of the endoprosthesis. Chemical transformation can includeoxidation/reduction, hydrolysis, substitution, and/or additionreactions, or other chemical reactions of the material from which theendoprosthesis, or a portion thereof, is made. The erosion can be theresult of a chemical and/or biological interaction of the endoprosthesiswith the body environment, e.g., the body itself or body fluids, intowhich the endoprosthesis is implanted and/or erosion can be triggered byapplying a triggering influence, such as a chemical reactant or energyto part or all of the endoprosthesis, e.g., to increase a reaction rate.For example, an endoprosthesis, or a portion thereof, can be formed froman active metal, e.g., Mg or Ca or an alloy thereof, and which can erodeby reaction with water, producing the corresponding metal oxide andhydrogen gas (a redox reaction). For example, an endoprosthesis, or aportion thereof, can be formed from an erodible or bioerodible polymer,an alloy, and/or a blend of erodible or bioerodible polymers which canerode by hydrolysis with water. The erosion occurs to a desirable extentin a time frame that can provide a therapeutic benefit. For example, inembodiments, the endoprosthesis exhibits substantial mass reductionafter a period of time when a function of the endoprosthesis, such assupport of the lumen wall or drug delivery, is no longer needed ordesirable. In particular embodiments, the endoprosthesis exhibits a massreduction of about 10 percent or more, e.g. about 50 percent or more,after a period of implantation of one day or more, e.g. about 60 days ormore, about 180 days or more, about 600 days or more, or 1000 days orless. In embodiments, only portions of the endoprosthesis exhibitserodibility. For example, an exterior layer or coating may benon-erodible, while an interior layer or body is erodible. In someembodiments, the endoprosthesis includes a non-erodible coating or layerof a radiopaque material, which can provide long-term identification ofan endoprosthesis location.

Erosion rates can be measured with a test endoprosthesis suspended in astream of Ringer's solution flowing at a rate of 0.2 ml/second. Duringtesting, all surfaces of the test endoprosthesis can be exposed to thestream. For the purposes of this disclosure, Ringer's solution is asolution of recently boiled distilled water containing 8.6 gram sodiumchloride, 0.3 gram potassium chloride, and 0.33 gram calcium chlorideper liter of solution.

Other aspects, features and advantages will be apparent from thedescription of the preferred embodiments thereof and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is a perspective view of an embodiment of an endoprosthesis.

FIG. 1B is an enlarged cross-sectional view of the endoprosthesis ofFIG. 1A.

FIG. 2A is an enlarged perspective view of a portion of anendoprosthesis.

FIG. 2B is an enlarged cross-sectional view of and embodiment of theportion of the endoprosthesis of FIG. 2A.

FIG. 3A is an enlarged perspective view of a portion of anendoprosthesis.

FIG. 3B is an enlarged cross-sectional view of an embodiment of theportion of the endoprosthesis of FIG. 3A.

FIG. 3C is an enlarged cross-sectional view of an embodiment of theportion of the endoprosthesis of FIG. 3A.

FIG. 3D is an enlarged cross-sectional view of an embodiment of theportion of the endoprosthesis of FIG. 3A.

FIG. 4A is an enlarged perspective view of a portion of anendoprosthesis.

FIG. 4B is an enlarged perspective view of an embodiment of the portionof the endoprosthesis of FIG. 4A.

FIG. 5A is an enlarged perspective view of a portion of anendoprosthesis.

FIG. 5B is an enlarged perspective view of an embodiment of the portionof the endoprosthesis of FIG. 5A.

FIG. 6A is an enlarged perspective view of a portion of anendoprosthesis.

FIG. 6B is an enlarged perspective view of an embodiment of the portionof the endoprosthesis of FIG. 6A.

FIG. 7A is an enlarged perspective view of a portion of anendoprosthesis.

FIG. 7B is an enlarged perspective view of an embodiment of the portionof the endoprosthesis of FIG. 7A.

FIG. 8A is an enlarged cross-sectional view of a portion of anendoprosthesis.

FIG. 8B is an enlarged cross-sectional view of an embodiment of theportion of the endoprosthesis of FIG. 8A.

FIG. 9A is an enlarged perspective view of a portion of anendoprosthesis.

FIG. 9B is an enlarged perspective view of an embodiment of the portionof the endoprosthesis of FIG. 9A.

FIG. 10A is an enlarged cross-sectional view of an embodiment of aportion of an endoprosthesis.

FIG. 10B is an enlarged cross-sectional view of an embodiment of theportion of the endoprosthesis of FIG. 10A.

FIG. 11A is an enlarged cross-sectional view of a portion of anendoprosthesis.

FIG. 11B is an enlarged cross-sectional view of an embodiment of aportion of the endoprosthesis of FIG. 11A.

FIG. 11C is an enlarged cross-sectional view of an embodiment of aportion of an endoprosthesis of FIG. 11A.

FIG. 12A is a perspective view of an embodiment of an endoprosthesis.

FIG. 12B is an enlarged cross-sectional view of the endoprosthesis ofFIG. 12A.

FIG. 12C is an enlarged cross-sectional view of a portion of theendoprosthesis of FIG. 12B.

FIG. 12D is an enlarged cross-sectional view of the endoprosthesis ofFIG. 12A.

FIG. 12E is an enlarged cross-sectional view of a portion of theendoprosthesis of FIG. 12D.

FIG. 13A is a perspective view of an embodiment of an endoprosthesis.

FIG. 13B is an enlarged cross-sectional view of the endoprosthesis ofFIG. 13A.

FIG. 13C is an enlarged cross-sectional view of a portion of theendoprosthesis of FIG. 13B.

FIG. 13D is an enlarged cross-sectional view of the endoprosthesis ofFIG. 13A.

FIG. 13E is an enlarged cross-sectional view of a portion of theendoprosthesis of FIG. 13D.

FIG. 14 is an enlarged cross-sectional view of an embodiment of aportion of an endoprosthesis.

FIG. 15 is a sequence illustrating a method of making an endoprosthesis.

FIG. 16 is an enlarged cross-sectional view of an embodiment of aportion of an endoprosthesis.

FIG. 17 is a perspective view of an embodiment of an endoprosthesis.

FIG. 18 is a perspective view of an embodiment of an endoprosthesis.

FIG. 19 is a sequence illustrating a method of making an endoprosthesis.

FIG. 20A is a perspective view of an embodiment of an endoprosthesis.

FIG. 20B is an enlarged cross-sectional view of the endoprosthesis ofFIG. 20A.

FIG. 20C is an enlarged cross-sectional view of a portion of theendoprosthesis of FIG. 20B.

FIG. 21 is a perspective view of an embodiment of an endoprosthesis.

FIG. 22 is a perspective view of an embodiment of an endoprosthesis.

DETAILED DESCRIPTION

Referring to FIGS. 1A and 1B, an endoprosthesis 2 includes a pluralityof generally circumferential struts 6 and connecting struts 8. Thecircumferential struts 6 can be directly interconnected to one anotherand/or they can be connected by connecting struts 8. The endoprosthesiscan be delivered into a body lumen, such as a vasculature, in a reduceddiameter configuration and then expanded into contact with the lumenwall to, e.g., maintain patency at the site of an occlusion.

Referring as well to FIGS. 2A and 2B, a perspective cross-sectional viewthrough a strut 6, 8, the strut is formed of a body 4 made of abioerodible material, e.g., a metal such as magnesium. The strut alsoincludes an erosion modifying material in layers 16 on the surface ofthe body 4. The erosion modifying material modifies the rate at whichportions of the body 4 are eroded when the stent is placed in the lumen.In embodiments, the erosion modifying material is a non-erodiblematerial or an erodible material that erodes at a different rate thanthe erodible material forming the body 4 so as to form a barrier thatreduces or prevents exposure of the body 4 to physiological body fluids.For example, the erosion modifying material can be a ceramic such as anoxide of the erodible material forming the body 4 (e.g., magnesiumoxide). The erosion modifying material can include a therapeutic drug.

The pattern of the erosion modifying material on the surface of thebody, and the geometry and dimensions of the body are selected so that adesirable erosion geometry forms as the body erodes. In particular, thegeometry of the eroding body can be selected to maintain the mechanicalstrength of the strut, even after substantial erosion, and to reducepremature fragmentation or fragmentation into large pieces.

Continuing to refer to FIGS. 2A and 2B, the strut 6, 8 extends in adirection along a Z-axis generally perpendicular to a plane on an X-Yaxis. The cross-section or strut dimensions in the X-Y plane aregenerally smaller than along the Z-axis. In the illustrated embodiment,the strut has an initial rectangular geometry with an abluminal surface10 (e.g., an exterior surface 10), an adluminal surface 12 (e.g., aninterior surface 12) and two side surfaces 14. The strut has an initialthickness T between the surfaces 10 and 12 and an initial width W beforethe stent is implanted. The erosion modifying material is provided incorresponding coextensive layers 16 on the abluminal and adluminalsurfaces of the body, which inhibit erosion from those surfaces. In someembodiments, each layer 16 can include a plurality of layers, which caninclude the same or different materials. Within each layer, thecomposition can include the same or different materials at differentportions of the layer.

FIGS. 3A and 3B illustrate the strut after erosion upon implantation ina body lumen. Erosion of the body 4 occurs primarily at the sidesurfaces, yielding a I-beam geometry where the top and bottom of the Icorrespond to the abluminal and adluminal surfaces, respectively. TheI-beam geometry provides mechanical strength to the strut even though asubstantial amount of the body 4, e.g., 50% or more of the area in theX-Y plane has been eroded. Moreover, at least one dimension of thestrut, the vertical line of the I corresponding to the strut thicknessis maintained. The geometry of the I-beam can result as a function ofsurface diffusion and/or mass transport processes of the erodingendoprosthesis.

Referring as well to FIG. 3C, after further erosion, e.g., 75 percent ormore of the strut has eroded, the I-beam geometry is still maintained.Referring as well to FIG. 3D, after 90 percent or more of the strut haseroded, the I-beam geometry is broken, and the erosion modifying layeris itself eroded. The erosion modifying layer reduces the likelihoodthat loose fragments of the body 4 will break off; the slow erosion ofthe strut also provides time for endothelialization prior to completeerosion.

Erosion to a desirable geometry can be controlled by selecting theinitial dimensions and geometry of the bioerodible body, and the patternand nature of the erosion control material. The bioerodible bodypreferably has a geometry such that the ratio of the characteristicdimensions in the X-Y direction is between about 2:1 to 1:2, e.g., about1:1. For example, for a strut with a rectangular cross section, theratio of the thickness to the width is about 2:1 to about 1:2. For astrut with a circular cross section, the ratio of its radii or diametersin the cross section is 1:1. As discussed above, the erosion modifyinglayer can include a biodegradable and/or non-biodegradable secondmaterial with a lower erosion rate than a first material of abioerodible body of an endoprosthesis. Examples are ceramics, metals orpolymers, which provide a barrier that reduces the exposure of theerodible body to fluids by requiring diffusion of body fluids throughthe erosion modifying layer or preventing exposure of the erodible bodyto body fluids. In some embodiments, the erosion modifying layer caninclude a plurality of layers, which can include the same or differentmaterials. Within each layer, the composition can include the same ordifferent materials at different portions of the layer. In someembodiments, the erosion rate of the erosion modifying layer is fromabout 10% (e.g., from about 25%, from about 50%, from about 150%, fromabout 200 percent, from about 400 percent, from about 600 percent, fromabout 8000%) less than the erosion rate of a bioerodible body to about1000% (e.g., to about 800%, to about 600%, to about 400%, to about 200%,to about 150%) less than the erosion rate of a bioerodible body. In someembodiments, the erosion rate of erosion modifying layer can range fromabout 0.001% (e.g., from about 0.01%, from about 0.1%, from about 0.5%)to about 1% (e.g., to about 0.5%, to about 0.1%, to about 0.01%) of theinitial mass of that portion per day. The erosion rate of a bioerodiblebody can range from about 0.2% (e.g., from about 0.5%, from about 1%,from about 2%) to about 5% (e.g., to about 2%, to about 1%, to about0.5%) of the initial mass of that portion per day. In some embodimentsin which the erosion modifying layer includes a non-biodegradable secondmaterial, the second material is radiopaque and can provide long termidentification of the endoprosthesis location (e.g., by x-ray, MRI)within a body. In some embodiments, the erosion modifying layer includesstainless steel, which can promote endothelialization of theendoprosthesis and/or reduce thrombus risk.

The thickness of the erosion modifying material can be selected tocontrol the rate of exposure of the erodible material to body fluid. Thethickness can be uniform, variable in a gradient manner, variable in astepwise manner, and/or variable in a random manner along a length or awidth of an endoprosthesis. The thickness of the layers can range fromabout 1 nm (e.g., from about 5 nm, from about 25 nm, from about 100 nm,from about 500 nm, from about 800 nm, from about 1 μm, from about 2 μm,from about 3 μm, from about 4 μm, from about 5 μm, from about 6 μm, fromabout 7 μm, from about 8 μm, from about 9 μm) to about 10 μm (e.g., toabout 9 μm, to about 8 μm, to about 7 μm, to about 6 μm, to about 5 μm,to about 4 μm, to about 3 μm, to about 2 μm, to 1 μm, to about 800 nm,to about 500 nm). The thickness of an erosion modifying layer can alsobe expressed as a fraction of a thickness of a bioerodible body. Forexample, the thickness of the erosion modifying layer can be at mostabout 50% (e.g., at most about 40%, at most about 30%, at most about20%, at most about 10%, at most about 5%, at most about 2%, at mostabout 1%) the thickness of the bioerodible body and/or at least about0.001%, (e.g., at least about 0.004%, at least about 0.01%, at leastabout 0.1%, at least about 1%, at least about 2%, at least about 5%, atleast about 10%, at least about 25%) the thickness of the bioerodiblebody.

A strut can erode in a variety of erosion patterns and/or geometries.For example, referring to FIG. 4A, in some embodiments a strut 6′, 8′has an initial rectangular geometry with an erodible body 4′, anabluminal surface 10′, an adluminal surface 12′ and two side surfaces14′. The strut has an initial thickness T′ between the surfaces 10′ and12′ and an initial width W′ before the stent is implanted. Erosionmodifying layers 16′ are provided in corresponding coextensive layers onthe side surfaces of the body, which inhibit erosion from thosesurfaces. FIG. 4B illustrates the strut after partial erosion, uponimplantation in a body lumen. Erosion of body 4′ occurs primarily at theabluminal and adluminal surfaces, resulting in a geometry as shown inFIG. 4B. Thickness T′ decreases from the side surfaces toward the centerof the strut. However, the width W′ is maintained even though asubstantial amount of the body 4′, e.g., 50% or more of the area in theX-Y plane, has been eroded. The erosion geometry can provide mechanicalstrength to the eroded strut.

In some embodiments, erosion modifying layers are providedintermittently along the perimeter of a strut. As an example, as shownin FIG. 5A, layers 22 are provided at the four edges of strut 20. Asshown in FIG. 5B, upon implantation, erosion of body 24 occurs startingat the uncoated side surfaces, resulting in a X-shaped geometry at theX-Y plane. As an example, as shown in FIG. 6A, erosion modifying layers32 are provided at opposing surfaces 35, 36 including the four edges,and intermittently at the remaining surfaces 38, 39 of strut 30. Uponimplantation, as shown in FIG. 6B, partial erosion of body 34 occurs atthe exposed surfaces, resulting in a strut having grooves along theabluminal and adluminal surfaces. Referring to FIG. 7A, erosionmodifying layers 42 are provided intermittently along the surfaces ofstrut 40, which result in erosion of body 44 starting at the exposedsurfaces upon implantation in a body lumen. Referring to FIG. 7B, aftererosion, the strut has a series of grooves forming an interdigitatedpattern. The erosion geometry of struts 20, 30, 40 in FIGS. 5B, 6B, 7Bcan provide mechanical strength to the eroded strut.

In some embodiments, as shown in FIG. 8A, a strut 50 has an initialcircular geometry at the X-Y plane with initial radius and/or diameter.An erosion modifying layer 52 can intermittently coat the strut surfacealong the perimeter of the circular strut. Upon implantation, erosionstarting at the uncoated surface 54 of strut 50 can result in a groovedgeometry that maintains the initial radii in the lobe regions, forexample, as shown in FIG. 8B. Referring to FIGS. 9A and 9B, in someembodiments, a strut 60 is coated with an erosion modifying layer 62.Erosion modifying layer 62 can curve along the Z-axis, for example, in asinusoidal pattern. Upon implantation, erosion of an erodible body 64starting at the uncoated surfaces 66 can result in a curved geometryalong the Z axis. Further erosion can result in a I-beam geometry orgrooved geometry.

In some embodiments, a strut can erode to generate a bioerodible bodywith, for example, an overall grooved geometry at the X-Y plane, anoverall rectangular geometry at the X-Y plane, or combinations thereof.In some embodiments, an erosion modifying layer can coat two or moreadjoining surfaces of a strut, or two or more non-adjoining surfaces ofa strut. An endoprosthesis having a strut with one or more surfacescovered by an erosion modifying layer can, for example, have a lowererosion rate, maintain structural integrity for a longer duration, limitthe degrees of freedom available for erosion, and reduce the riskassociated with penetrating localized erosion and attendantfragmentation. Referring to FIGS. 10A and 11A, a strut 70, 80 coatedwith erosion modifying layers 72, 82 on three surfaces can erode togenerate a bioerodible body 74, 84 having, for example, an overallgrooved or concave geometry at the X-Y plane (e.g., FIG. 10B), or anoverall rectangular cross-section (FIGS. 11B and 11C), or combinationsthereof.

The erosion modifying layer can have a uniform thickness along thelength of the endoprosthesis, or the erosion modifying layer can have avariable thickness distribution, which can tailor the rate anddirectionality of endoprosthesis erosion. In certain embodiments, anerosion modifying layer can have variable thickness throughout thelength of the endoprosthesis. For example, as shown in FIGS. 12A, 12B,12C, 12D, and 12E, an erosion modifying layer 96 can be thicker on afirst end 92 of an endoprosthesis 90 and decrease gradually in thicknesstoward a second, opposite end 94 of endoprosthesis 90, thus allowing thesecond end of the endoprosthesis to erode before the first end. Layers96 can be the same or different. As another example, as shown in FIGS.13A, 13B, 13C, 13D, and 13E, an erosion modifying layer 108 can bethicker at a middle portion 106 of an endoprosthesis 100 than at theends 102 and 104 of the endoprosthesis, thus allowing the ends of theendoprosthesis to erode before the middle of the endoprosthesis. Layers108 can be the same or different. In some embodiments, the thicknessesof an erosion modifying layer at different surfaces on the strut can bethe same or different. For example, to compensate for any difference inerosion rates between an interior surface and an exterior surface and toallow a cross-section of an endoprosthesis to erode relatively uniformlyat the bioerodible body, an erosion modifying layer located at theinterior may be thicker than a layer located at the exterior along thecross section of the endoprosthesis. In some embodiments, the thicknessof an erosion modifying layer can change along a width of the strut. Asshown in FIG. 14, a strut 110 with two opposite side surfaces 112 and114 coated with an erosion modifying layer 116 can have increasingthicknesses of the erosion modifying layer from an abluminal surface 118to an interior surface 119. Layers 116 can be the same or different. Insome embodiments, each of layers 96, 108, and 116 can include aplurality of layers, which can include the same or different materials.In some embodiments, within each layer, the composition can include thesame or different materials at different portions of the layer.

An endoprosthesis can have struts having a rectangular cross-section, asquare cross-section, a circular cross-section, an ovaloidcross-section, an elliptical cross-section, a polygonal cross-section(e.g., a hexagonal, an octagonal cross-section), or an irregularlyshaped cross-section. The endoprosthesis can have an erosion modifyinglayer covering a portion of a total surface area of the endoprosthesis.In some embodiments, an erosion modifying layer covers at most 99percent (e.g., at most about 90 percent, at most about 80 percent, atmost about 70 percent, at most about 60 percent, at most about 50percent, at most about 40 percent, at most about 30 percent, at mostabout 20 percent) and/or at least about 10 percent (e.g., at least about20 percent, at least about 30 percent, at least about 40 percent, atleast about 50 percent, at least about 60 percent, at least about 70percent, at least about 80 percent) of a total surface area of anendoprosthesis.

In some embodiments, the erosion modifying layer can cover the entiresurface of the endoprosthesis. The erosion modifying layer can include aplurality of layers, the composition of the layers can be the same ordifferent. Within each layer, the composition can include the same ordifferent materials at different portions of the layer. Depending on thematerials in the erosion modifying layer(s), the erosion process can betailored to follow a desired sequence. For example, one or more erosionmodifying layers located at select portions of the endoprosthesis (e.g.,the side surfaces of a strut) can include a more erodible material(s)that erodes prior to the remaining layers (e.g., located at theabluminal and adluminal surfaces of a strut), which can include a lesserodible material(s). The erosion sequence can expose the endoprosthesisto body fluids at different locations and/or at different times duringthe lifetime of the endoprosthesis, which can produce a desired erosiongeometry (e.g., an I-beam geometry).

In some embodiments, the erosion modifying layer and/or the bioerodiblebody have pores and/or patterns to adjust the erosion rate and/orerosion location of an endoprosthesis. As an example, an erosionmodifying layer with open or closed pores extending throughout the layercan erode at a faster rate than a solid layer and/or allow the diffusionof body fluids through the erosion modifying layer, which can in turnallow the bioerodible body to erode at a faster rate. Pores can range involume from about 500 nm³ (e.g., from about 0.00005 μm³, from about0.0005 μm³, from about 0.005 μm³, from about 0.05 μm³, from about 0.5μm³, from about 1 μm³, from about 5 μm³, from about 35 μm³, or fromabout 50 μm³) to about 550 μm³ (e.g., to about 450 μm³, to about 300μm³, to about 200 μm³, to about 100 μm³, to about 75 μm³, to about 40μm³, to about 10 μm³, to about 5 μm³, to about 1 μm³, to about 0.5 μm³,to about 0.05 μm³, to about 0.005 μm³, or to about 0.00005 μm³). Asanother example, a bioerodible endoprosthesis coated with a patternederosion modifying layer can preferentially erode at certain exposedlocations and can have controlled erosion geometries. A pattern includesa repeating sequence of one or more shapes or motifs, for example,grids, squares, circles, and/or lines. In some embodiments, anendoprosthesis having a patterned erosion modifying layer has enhancedendothelialization and reduced thrombus in a body lumen.

An erosion modifying layer located on the abluminal, adluminal, or theside surface of the strut can have the same chemical composition ordifferent compositions. For example, an adluminal surface (e.g., FIG. 2,surface 12) can contact bodily fluid more than an abluminal surface(e.g., FIG. 2, surface 10), which can contact a wall of a bodypassageway, and as a result, the interior surface can erode more quicklythan the exterior surface. To compensate for the difference in erosionand to allow a given cross-section of an endoprosthesis to eroderelatively uniformly, the interior surface can have a layer having achemical composition that erodes more slowly than the chemicalcomposition of a layer at the exterior surface.

In some embodiments, the erosion rate of an endoprosthesis is tailoredby changing the percentage of cold working of a metal or an alloy.Without being bound by theory, it is believed that cold workingincreases the susceptibility to erosion of a material by inducingdislocations and other defects in the structure, which tend to be anodicand corrode. For example, a bioerodible body can be cold-worked at ahigher percentage than an erosion modifying layer so that thebioerodible body can erode before an erosion modifying layer.

Referring to FIG. 15, a method 200 of making an endoprosthesis asdescribed herein is shown. Method 200 includes forming a bioerodibletube (step 202), forming a pre-endoprosthesis from the bioerodible tube(step 204), and applying one or more erosion modifying layers to thepre-endoprosthesis (step 206) to form an endoprosthesis. In someembodiments, one or more erosion modifying layers are applied to thebioerodible tube, and the tube with the applied erosion modifyinglayer(s) is subsequently formed into an endoprosthesis.

The bioerodible tube can be formed (step 202) by manufacturing a tubularmember including (e.g., is formed of) one or more bioerodible materialsand capable of supporting a bodily lumen. For example, a mass ofbioerodible material can be machined into a rod that is subsequentlydrilled to form the tubular member. As another example, a sheet ofbioerodible material can be rolled to form a tubular member withoverlapping portions, or opposing end portions of the rolled sheet canbe joined (e.g., welded) together to form a tubular member. Abioerodible material can also be extruded to form a tubular member. Incertain embodiments, a bioerodible tube can be made by thermal spraying,powder metallurgy, thixomolding, die casting, gravity casting, and/orforging. The bioerodible or erodible material can be a substantiallypure metallic element, an alloy, or a composite. Examples of metallicelements include iron, magnesium, zinc, and alloys thereof. Examples ofalloys include iron alloys having, by weight, 88-99.8% iron, 0.1-7%chromium, 0-3.5% nickel, and less than 5% of other elements (e.g.,magnesium and/or zinc); or 90-96% iron, 3-6% chromium and 0-3% nickelplus 0-5% other metals. Other examples of alloys include magnesiumalloys, such as, by weight, 50-98% magnesium, 0-40% lithium, 0-5% ironand less than 5% other metals or rare earths; or 79-97% magnesium, 2-5%aluminum, 0-12% lithium and 1-4% rare earths (such as cerium, lanthanum,neodymium and/or praseodymium); or 85-91% magnesium, 6-12% lithium, 2%aluminum and 1% rare earths; or 86-97% magnesium, 0-8% lithium, 2%-4%aluminum and 1-2% rare earths; or 8.5-9.5% aluminum, 0.15%-0.4%manganese, 0.45-0.9% zinc and the remainder magnesium; or 4.5-5.3%aluminum, 0.28%-0.5% manganese and the remainder magnesium; or 55-65%magnesium, 30-40% lithium and 0-5% other metals and/or rare earths.Magnesium alloys are also available under the names AZ91D, AM50A, andAE42. Other erodible materials are described in Bolz, U.S. Pat. No.6,287,332 (e.g., zinc-titanium alloy and sodium-magnesium alloys);Heublein, U.S. Patent Application 2002000406; and Park, Science andTechnology of Advanced Materials, 2, 73-78 (2001), all of which arehereby incorporated by reference herein in their entirety. Inparticular, Park describes Mg—X—Ca alloys, e.g., Mg—Al—Si—Ca, Mg—Zn—Caalloys. Other suitable alloys include strontium. As an example,strontium can be a component in a magnesium alloy. The bioerodible tubecan include more than one bioerodible material, such as differentbioerodible materials physically mixed together, multiple layers ofdifferent bioerodible materials, and/or multiple sections of differentbioerodible materials along a direction (e.g., length) of the tube. Anexample of a composite is as a mixture of a magnesium alloy in abioerodible polymer, in which two or more distinct substances (e.g.,metals, ceramics, glasses, and/or polymers) are intimately combined toform a complex material.

As shown in FIG. 15, after the bioerodible tube is formed, the tube isformed into a pre-endoprosthesis (step 204). In some embodiments,selected portions of the tube can be removed to form circular andconnecting struts (e.g., 6, 8) by laser cutting, as described in U.S.Pat. No. 5,780,807, hereby incorporated by reference in its entirety.Other methods of removing portions of the tube can be used, such asmechanical machining (e.g., micro-machining, grit blasting or honing),electrical discharge machining (EDM), and photoetching (e.g., acidphotoetching). The pre-endoprosthesis can be etched and/orelectropolished to provide a selected finish. In certain embodiments,such as jelly-roll type endoprostheses, step 204 is omitted.

Next, the erosion modifying layer(s) is applied to thepre-endoprosthesis (step 206) to form an endoprosthesis. Prior toapplying the erosion modifying layer, selected surfaces (e.g., interiorsurface) or portions (e.g., portion between the end portions of theendoprosthesis) of the pre-endoprosthesis can be masked so that theerosion modifying layer will not be applied to the masked surfaces orportions. In some embodiments, prior to applying the erosion modifyinglayer, pores can be formed on the pre-endoprosthesis (e.g., by micro-arcsurface modification, sol-gel templating processes, near net shape alloyprocessing technology such as powder injection molding, adding foamingstructures into a melt or liquid metal, melting a powder compactcontaining a gas evolving element or a space holder material,incorporating a removable scaffold (e.g., polyurethane) in a metalpowder/slurry prior to sintering, sintering hollow spheres, sinteringfibers, combustion synthesis, powder metallurgy, bonded fiber arrays,wire mesh constructions, vapor deposition, three-dimensional printing,and/or electrical discharge compaction). In some embodiments, pores canbe formed by incorporating embedded microparticles and/or compounds(e.g., a salt) within the antioxidant layer (e.g., a polymerizablemonomer, a polymer, a metal alloy), forming the antioxidant layer, andremoving (e.g., dissolving, leaching, burning) the microparticles and/orcompounds to form pores at locations where the microparticles and/orcompounds were embedded. Removable (e.g., dissolvable) microparticlescan be purchased, for example, from MicroParticles GmbH. In someembodiments, pores are formed by using a gas as a porogen, bondingfibers, and/or phase separation in materials such as polymers, metals,or metal alloys.

Suitable erosion modifying layer materials can include a polymerincluding covalently bound C, N, O, and halogen, a ceramic material, anoxide, a carbide, a halide, a metal, a metallic alloy, and/or ametal-containing polymer. For example, suitable polymers includebioerodible polymers as polylactic acid (PLA), polylactic glycolic acid(PLGA), polyanhydrides (e.g., poly(ester anhydride)s, fatty acid-basedpolyanhydride, amino acid-based polyanhydride), polyesters,polyester-polyanhydride blends, polycarbonate-polyanhydride blends,and/or combinations thereof. Suitable ceramic materials include, forexample, iridium oxide. Suitable oxides include magnesium oxide,titanium oxide, and/or aluminum oxide. Suitable nitrides includemagnesium nitride, titanium nitride, titanium oxynitride, iron nitride,and/or silicon nitride. Suitable carbides include iron carbide andsilicon nitride. Suitable halides include magnesium fluoride. Suitablemetals and/or a metallic alloys include stainless steel, titanium,niobium, a radiopaque metal such as gold, platinum, iridium, and alloysthereof; an alloy such as bioerodible magnesium alloys and iron alloysas previously described having adjusted compositions so that erosionoccurs at a different rate than the bioerodible body. Suitable inert ordissolvable polymers including metals (e.g., Fe, Au, Pt) or metalcompounds such as organometallic complexes. Depending on the erosionmodifying layer material, one or more material can be dissolved in asolvent and applied to the pre-endoprosthesis, and/or two or moredifferent materials can be blended together in the form of, for example,a composite such as a metal matrix composite (e.g., in a manner that onematerial is embedded or encapsulated in a remaining material) andapplied to the pre-endoprosthesis. In some embodiments, for example,erosion modifying coatings are generated by physical or plasma vapordeposition, thermal metal spraying, dip coating, electrostatic spraying,conventional air atomization spraying, ion implantation (e.g., by plasmaimmersion ion implantation, by laser-driven ion implantation),electrochemical deposition, oxidation (e.g., anodizations), chemicalgrafting, interlayer transitional coatings to bond multiple layers,and/or metallurgical augmentation (e.g., peening, localizedmetallurgical treatments). In some embodiments, pores are generated inan erosion modifying layer, e.g., by powder injection molding sol-geltemplating processes, near net shape alloy processing technology such aspowder injection molding, micro-arc surface modification, sol-geltemplating processes, adding foaming structures into a melt or liquidmetal, melting a powder compact containing a gas evolving element or aspace holder material, incorporating a removable scaffold (e.g.,polyurethane) in a metal powder/slurry prior to sintering, sinteringhollow spheres, sintering fibers, combustion synthesis, powdermetallurgy, bonded fiber arrays, wire mesh constructions, vapordeposition, three-dimensional printing, and/or electrical dischargecompaction). In some embodiments, pores can be formed by incorporatingembedded microparticles and/or compounds (e.g., a salt) within theantioxidant layer (e.g., a polymerizable monomer, a polymer, a metalalloy), forming the antioxidant layer, and removing (e.g., dissolving,leaching, burning) the microparticles and/or compounds to form pores atlocations where the microparticles and/or compounds were embedded.Removable (e.g., dissolvable) microparticles can be purchased, forexample, from MicroParticles GmbH. In some embodiments, pores are formedby using a gas as a porogen, bonding fibers, and/or phase separation inmaterials such as polymers, metals, or metal alloys. In certainembodiments, patterns are generated in an erosion modifying layer, e.g.,by laser ablation, lithography, ink-jet printing, and/or screenprinting.

In some embodiments, a medicament is incorporated into an erosionmodifying coating on an endoprosthesis. For example, a medicament can beadsorbed onto an erosion modifying coating on an endoprosthesis. Amedicament can be encapsulated in a bioerodible material and embedded inan erosion modifying coating on an endoprosthesis. As another example, amedicament can be dissolved in a polymer solution and coated onto anendoprosthesis. Incorporation of a medicament is described in U.S. Ser.No. 10/958,435 filed Oct. 5, 2004, hereby incorporated by reference.

In some embodiments, an endoprosthesis can have greater than one type oferosion modifying coating located at the same or different locations onthe endoprosthesis. Referring to FIG. 16, as an example, anendoprosthesis can have a polymer coating 210 superimposed upon astainless steel coating 212 on a strut 214. As another example, anendoprosthesis can have a ceramic coating on an exterior surface, and apolymer coating on an interior surface of a strut. In certainembodiments, an erosion modifying layer can be applied to apre-endoprosthesis in one layer, or in multiple layers (e.g., at leasttwo layers, at least three layers, at least four layers, at least fivelayers) in order, for example, to provide greater control over thethickness of an erosion modifying layer. Within an erosion modifyinglayer, the thickness and composition of a second material can be thesame or different to provide desired erosion rates and erosion sequence.For example, the intermediate portion of an endoprosthesis can have asmaller thickness of a non-bioerodible second material than the endportions of the endoprosthesis, which can contain a greater thickness ofa bioerodible second material. The erosion modifying layers can beapplied the same way or in different ways. For example, a first,innermost erosion modifying layer can be plasma-deposited on thepre-endoprosthesis, and a second, outer erosion modifying layer caninclude a polymer that is dip-coated onto the first layer.

In some embodiments, an erosion modifying coating partially coats one ormore portions of an endoprosthesis. Referring to FIG. 17, as an example,an endoprosthesis 220 can have a band(s) 222 of the same or differentcoatings along the length of the endoprosthesis. As shown in FIG. 18, asan example, an endoprosthesis 230 can have a strip(s) of the same ordifferent coatings along the circumference of the endoprosthesis. Bandsand strips can be coated onto the endoprosthesis by selectively maskingcertain areas of the endoprosthesis. Bands and strips of erosionmodifying coating can have pore/patterns, and/or have differentthicknesses as discussed above.

Referring now to FIG. 19, an endoprosthesis 300 having an increasingnumber of different erosion modifying layers along its length can beproduced from a metallic pre-endoprosthesis 240 by masking selectiveportions of the endoprosthesis. For example, during production, allportions of the pre-endoprosthesis can be coated with a first erosionmodifying layer to generate a pre-endoprosthesis 250. Next, a portion252 of the pre-endoprosthesis is masked (e.g., with a protectivepolymeric coating such as a styrene-isoprene-butadiene-styrene (SIBS)polymer), which protects the masked portion from further erosionmodifying layer coating, and the remaining section is coated with asecond erosion modifying layer to make a pre-endoprosthesis 270.Finally, a second portion 272 of the pre-endoprosthesis is masked, andthe remaining portion is further coated with a third erosion modifyinglayer to make pre-endoprosthesis 290. The protective coatings can beremoved, e.g., by rinsing in a solvent such as toluene to complete theproduction of endoprosthesis 300. An endoprosthesis having taperedthicknesses can be produced by masking the interior and/or outerportions with a movable sleeve and longitudinally moving the sleeveand/or the endoprosthesis relative to each other during implantation.

In some embodiments, the erosion modifying layer(s) can be applied tothe bioerodible tube prior to forming the bioerodible tube into anendoprosthesis (if necessary). As a result, the endoprosthesis can haveits exterior and interior surfaces coated with the erosion modifyinglayer(s), and the side surfaces of the endoprosthesis can be free of theerosion modifying layer(s). Prior to applying the erosion modifyinglayer(s), the interior surface or the exterior surface of thebioerodible tube can be masked to apply the erosion modifying layer(s)to only selected portion(s) of the tube.

As another example, while the endoprosthesis can have both exterior andinterior surfaces coated with a desired erosion modifying layermaterial, in other embodiments, one or more segments of anendoprosthesis have only the exterior surfaces or the interior surfacescoated with an erosion modifying layer having a second material.Exterior surfaces of a pre-endoprosthesis can be coated with a desiredsecond material, e.g., by placing a mandrel, a pin or a sleeve that issized to mate with the selected inner surface(s) of thepre-endoprosthesis so that during coating, the second material iseffectively blocked from entering interior surface of thepre-endoprosthesis. Such an endoprosthesis, after implantation, may havea cross-section that has only two materials: an exterior surface that iscoated with the second material, and an interior surface that has notbeen coated. Interior surfaces of a pre-endoprosthesis can be coatedwith a desired erosion modifying layer material, e.g., by placing apolymeric coating on selected outer surface(s) of the pre-endoprosthesisso that during coating the second material can coat only the interiorsurface(s) and is prevented from coating the exterior surfaces.Alternatively, exterior surfaces can be protected by placing thepre-endoprosthesis in a tight-fitting tube, e.g., a heat shrink tube, tocover the exterior surfaces. In some embodiments, photo-lithographyand/or stereo-lithography can be used to mask surfaces of apre-endoprosthesis to prevent coating of an erosion modifying layermaterial.

In use, the endoprostheses can be used, e.g., delivered and expanded,using a catheter delivery system, such as a balloon catheter system.Catheter systems are described in, for example, Wang U.S. Pat. No.5,195,969, Hamlin U.S. Pat. No. 5,270,086, and Raeder-Devens, U.S. Pat.No. 6,726,712. Endoprosthesis and endoprosthesis delivery are alsoexemplified by the Radius® or Symbiot® systems, available from BostonScientific Scimed, Maple Grove, Minn.

The endoprostheses described herein can be of a desired shape and size(e.g., coronary stents, aortic stents, peripheral vascular stents,gastrointestinal stents, urology stents, and neurology stents).Depending on the application, the stent can have a diameter of between,for example, 1 mm to 46 mm. In certain embodiments, a coronary stent canhave an expanded diameter of from about 2 mm to about 6 mm. In someembodiments, a peripheral stent can have an expanded diameter of fromabout 5 mm to about 24 mm. In certain embodiments, a gastrointestinaland/or urology stent can have an expanded diameter of from about 6 mm toabout 30 mm. In some embodiments, a neurology stent can have an expandeddiameter of from about 1 mm to about 12 mm. An abdominal aortic aneurysm(AAA) stent and a thoracic aortic aneurysm (TAA) stent can have adiameter from about 20 mm to about 46 mm.

While a number of embodiments have been described, the invention is notso limited. In some embodiments, the erosion rate of a bioerodiblematerial is increased by forming, for example, a galvanic couple that isexposed to body fluids or an electrolyte solution. For example, theerosion rate of a bioerodible material (e.g., a magnesium alloy) can beincreased by addition of one or more other materials such as iron,nickel, copper, and cobalt, and/or low level impurities such as gold,platinum, and iridium. Referring to FIGS. 20A, 20B, and 20C, anendoprosthesis 310 can have a strut 312, which can have a bioerodiblebody 322 having an inner portion 318, a center portion 320, an exteriorportion 316, and two erosion modifying layers 314. Depending on thecomposition and thicknesses of the portions, the endoprosthesis can beconfigured to erode sequentially from an interior portion to an exteriorportion, from an exterior surface to an interior surface, from a centerportion to the exterior and interior portions, or from the exterior andinterior portions to the center portion. This construction can allow theendoprosthesis to support the body vessel initially using the strengthof multiple layers, and to reduce in thickness over time (e.g., aftercells have endothelialized the endoprosthesis). The reduction inthickness can enhance the flexibility the endoprosthesis to better matchthe natural state of the body vessel. As another example, anendoprosthesis can have multiple alloy compositions along the length ofa bioerodible body. For example, an alloy composition having a greaterrate of erosion can be located at a first end of the bioerodible body,while an alloy composition having a smaller rate of erosion can belocated at a second end of the bioerodible body, such that the first enderodes at a faster rate than the second end. The erosion directionalitycan allow for increased maintenance of patency for certain locations(e.g., weakened locations) in a body vessel.

The endoprostheses described herein can be a part of a stent, a coveredstent or a stent-graft. For example, an endoprosthesis can includeand/or be attached to a biocompatible, non-porous or semi-porous polymermatrix made of polytetrafluoroethylene (PTFE), expanded PTFE,polyethylene, urethane, or polypropylene.

The endoprostheses described herein can include non-metallic structuralportions, e.g., polymeric portions. The polymeric portions can beerodible. The polymeric portions can be formed from a polymeric alloy.Polymeric stents have been described in U.S. patent application Ser. No.10/683,314, filed Oct. 10, 2003; and U.S. patent application Ser. No.10/958,435, filed Oct. 5, 2004, the entire contents of each is herebyincorporated by reference herein.

The endoprostheses can include a releasable therapeutic agent, drug, ora pharmaceutically active compound, such as described in U.S. Pat. No.5,674,242, U.S. Ser. No. 09/895,415, filed Jul. 2, 2001, U.S. Ser. No.11/111,509, filed Apr. 21, 2005, and U.S. Ser. No. 10/232,265, filedAug. 30, 2002. The therapeutic agents, drugs, or pharmaceutically activecompounds can include, for example, anti-thrombogenic agents,antioxidants, anti-inflammatory agents, anesthetic agents,anti-coagulants, and antibiotics. The therapeutic agent, drug, or apharmaceutically active compound can be dispersed in a polymeric coatingcarried by the endoprosthesis. The polymeric coating can include morethan a single layer. For example, the coating can include two layers,three layers or more layers, e.g., five layers. The therapeutic agentcan be a genetic therapeutic agent, a non-genetic therapeutic agent, orcells. Therapeutic agents can be used singularly, or in combination.Therapeutic agents can be, for example, nonionic, or they may be anionicand/or cationic in nature. An example of a therapeutic agent is one thatinhibits restenosis, such as paclitaxel. The therapeutic agent can alsobe used, e.g., to treat and/or inhibit pain, encrustation of theendoprosthesis or sclerosing or necrosing of a treated lumen. Any of theabove coatings and/or polymeric portions can be dyed or renderedradio-opaque.

The endoprostheses described herein can be configured for non-vascularlumens. For example, it can be configured for use in the esophagus orthe prostate. Other lumens include biliary lumens, hepatic lumens,pancreatic lumens, uretheral lumens and ureteral lumens.

Other configurations of endoprosthesis are also possible. Referring toFIG. 21, an endoprosthesis 330 can have a tubular body with slotsremoved from the tubular body, an erosion modifying layer(s) can becoated onto an exterior surface 332, an interior surface 334, or any ofthe side surfaces 336 of the endoprosthesis. Referring to FIG. 22, anendoprosthesis 340 can have a braided or woven tubular body made ofintertwining filaments 338. The endoprosthesis can be coated with anerosion modifying layer(s) on the exterior or the interior of thetubular body. In some embodiments, a braided endoprosthesis can includeerosion modifying layer-coated and uncoated filaments.

All references, such as patent applications, publications, and patents,referred to herein are incorporated by reference in their entirety.

Other embodiments are within the claims.

1. An endoprosthesis, comprising: a plurality of struts, the plurality of struts comprising a plurality of generally circumferential struts being interconnected to one another or connected by one or more connecting struts, wherein at least one strut of the plurality of struts comprises a bioerodible magnesium or a bioerodible magnesium alloy, the at least one strut having a longest dimension extending in a z-axis, the at least one strut having a cross-section in an X-Y plane perpendicular to the z-axis, wherein the at least one strut has an initial geometry in the X-Y plane characterized by initial dimensions, and an erosion modifying material provided on the surface of the at least one strut which controls erosion to form a predetermined geometry such that, after erosion of at least about 50 percent of the area of the at least one strut in the X-Y plane, at least one initial dimension of the initial geometry is maintained in the X-Y plane.
 2. The endoprosthesis of claim 1, wherein the initial dimension is maintained after erosion of at least about 75 percent of the area of the at least one strut in the X-Y plane.
 3. The endoprosthesis of claim 1, wherein the initial dimension maintained corresponds to a maximum dimension of the initial geometry.
 4. The endoprosthesis of claim 1, wherein the predetermined geometry is an X.
 5. The endoprosthesis of claim 1, wherein the predetermined geometry is an interdigitated geometry.
 6. The endoprosthesis of claim 1, wherein the predetermined geometry is a radially lobed structure.
 7. The endoprosthesis of claim 1, wherein the predetermined geometry is a convex structure.
 8. The endoprosthesis of claim 1, wherein the predetermined geometry is square or rectangular.
 9. The endoprosthesis of claim 1, wherein the predetermined geometry extends substantially the full extent of the strut in the Z direction.
 10. The endoprosthesis of claim 1, wherein the initial geometry is square or rectangular.
 11. The endoprosthesis of claim 1, wherein the initial geometry is circular, ovaloid or elliptical.
 12. The endoprosthesis of claim 1, wherein the ratio of maximum initial dimensions in the X-Y plane is between about 2:1 and about 1:2.
 13. The endoprosthesis of claim 1, wherein the erosion modifying material comprises a material selected from the group consisting of a polymer, a ceramic, an oxide, a metal, an alloy, and a composite.
 14. The endoprosthesis of claim 1 wherein the erosion modifying material is provided as a layer, and the layer has varying thickness.
 15. The endoprosthesis of claim 14 wherein the thickness varies in the X or Y direction.
 16. The endoprosthesis of claim 14 wherein the thickness varies in the Z direction.
 17. The endoprosthesis of claim 14 wherein the thickness varies along the length of the endoprosthesis.
 18. The endoprosthesis of claim 1 formed of a plurality of struts arranged in the general form of a tube.
 19. The endoprosthesis of claim 1, wherein the endoprosthesis is balloon expandable.
 20. An endoprosthesis, comprising: a strut comprising a bioerodible magnesium or a bioerodible magnesium alloy, the strut having a longest dimension extending in a z-axis, the strut having a cross-section in an X-Y plane perpendicular to the z-axis, wherein the strut has an initial geometry in the X-Y plane characterized by initial dimensions, and an erosion modifying material provided on the surface of the strut which controls erosion to form an I-shape in the X-Y plane after erosion of at least about 50 percent of the area of the strut in the X-Y plane.
 21. The endoprosthesis of claim 20, wherein the ends of the I correspond to abluminal and adluminal sides of the endoprosthesis.
 22. An endoprosthesis, comprising: a plurality of struts, the plurality of struts comprising a plurality of generally circumferential struts being interconnected to one another or connected by one or more connecting struts, wherein at least one strut of the plurality of struts comprises a bioerodible magnesium or a bioerodible magnesium alloy, at least one strut having an initial geometry in X, Y, and Z directions, the at least one strut having a longest dimension extending in the Z direction, the X, Y, and Z directions each being perpendicular to each other; and an erosion modifying material provided on the surface of the at least one strut which controls erosion to form a predetermined geometry such that, after erosion of at least about 50 percent of the at least one strut, at least two initial dimensions in the X, Y, and Z directions are maintained.
 23. The endoprosthesis of claim 1, wherein the bioerodible magnesium or the bioerodible magnesium alloy has an erosion rate of between about 0.2% and 5% of its initial mass per day. 